Hero was born as Han Jaejoon (한재준, 韓在俊) in Gongju, Chungcheongnam-do, South Korea. At a young age, Hero was given up for adoption by his birth mother to the Kims and his name was changed to Kim Jaejoong. When he was sixteen, he moved to Seoul by himself in order to take part in the auditions held by SM Entertainment. Life in Seoul proved to be hard for Hero and he had to take various odd jobs to come up with the ₩150,000 needed for the rent. He once took on a role as a soldier in the movie Taegeukgi Hwinallimyeo (태극기 휘날리며); with the money he acquired from the job he used the money to buy gum, which he sold to a female employee at a supermarket.

On December 26, 2003, he debuted with TVXQ at a BoA and Britney Spears showcase under the name a fortune teller foretold years before: YoungWoong JaeJoong. Youngwoong means “hero” in Korean, hence his international name, Hero. Jaejoong also had picked the name Hero because he wanted to be the hero of the music industry. Jaejoong was selected from an audition of 5000 people. Jaejoong won first place, both in best looking and best vocals. That made him the main vocal of TVXQ(now in JYJ).